It is known that low density polyethylene (LDPE) made by high-pressure polymerization of ethylene with free-radical initiators as well as homogeneous or heterogeneous linear low density polyethylene (LLDPE) and ultra low density polyethylene (ULDPE) made by the copolymerization of ethylene and α-olefins with metallocene or Ziegler coordination (transition metal) catalysts at low to medium pressures can be used, for example, to extrusion coat substrates such as paper board, paper, and/or polymeric substrates; to prepare extrusion cast film for applications such as disposable diapers and food packaging; and to prepare extrusion profiles such as wire and cable jacketing. However, although LDPE generally exhibits excellent extrusion processability and high extrusion drawdown rates, LDPE extrusion compositions lack sufficient abuse resistance and toughness for many applications. For extrusion coating and extrusion casting purposes, efforts to improve abuse properties by providing LDPE compositions having high molecular weights (i.e., having melt index, I2, less than about 2 g/10 min) are not effective since such compositions inevitably have too much melt strength to be successfully drawn down at high line speeds.
While LLDPE and ULDPE extrusion compositions offer improved abuse resistance and toughness properties and MDPE (medium density polyethylene) extrusion compositions offer improved barrier resistance (against, for example, moisture and grease permeation), these linear ethylene polymers exhibit unacceptably high neck-in and draw instability; they also exhibit relatively poor extrusion processability compared to pure LDPE. One proposal commonly used in the industry is to blend LDPE with LLDPE. With LDPEs currently used, large amounts (e.g. more than 60%) of LDPE must be used in order to achieve the required neck-in. In some circumstances, the availability of LDPE may be limited, or there may be other reasons for desiring a lower level of LDPE, without unduly increasing neck-in. It has been discovered that if two different LDPE resins are used, one having a density in the range of from 0.915 to 0.930 g/cm3, and a melt index of from 0.1 to 3 g/10 min, and another having a density in the range of from 0.915 to 0.930 g/cm3, and a melt index of from 5 to 15 g/10 min, that less of the total amount of LDPE (for example less than 50%) is needed to achieve the same neck-in. This also allows the LLDPE portion to be more carefully tailored to meet particular requirements such as sealability or toughness.
It is generally believed that neck-in and melt strength are inversely related. Thus, in references such as U.S. Pat. Nos. 5,582,923 and 5,777,155 to Kale et al. (each of which is hereby incorporated by reference in its entirety), adding LLDPE to improve physical toughness came at the expense of extrudability factors such as increased neck-in. Thus in the extrusion coating industry, current practice is to utilize lower melt index LDPE for extrusion on equipment with narrower die widths and relatively low maximum take off rates. Such low melt index autoclave LDPE resins provide low neck-in (less than about 2.5 inches (1.25 inch for each side)) and sufficient draw-down speed. This is typically with older equipment. Faster equipment, typically with wider die widths and improved internal deckling, is supplied with higher melt index autoclave LDPE, which unfortunately, tends to yield greater neck-in.
In the preferred practice of the present invention the neck-in is less than approximately two and a half inches (1.25″ per side) at a haul-off rate of approximately 880 feet/minute. The neck-in generally decreases with increasing haul-off rates, making neck-in particularly problematic when using older equipment which is limited in the haul off rates obtainable. The practical range of melt index is from about 3 to about 30 g/10 min in most coating applications, and the compositions of the present invention can cover this entire range. It is desirable that the maximum operating speed of the extrusion coating equipment not be limited by the properties of the resin being used. Thus it is desirable to use resin which exhibits neither draw instability nor breaking before the maximum line speed is reached. It is even more desirable that such resin exhibit very low neck-in, less than about 2.5 inches. The resins provided in this invention exhibit low neck-in and excellent draw stability while the draw-down capability required is obtained by selecting the correct melt index. Typically the melt index of the overall blend is in the range of 5-15 g/10 min. It is a further feature of this invention that it provides a resin composition at for example 8 MI that will be suitable for extrusion on both older equipment having slow take-off and modern high speed equipment. In both situations the neck-in can be less than 2.5 inches.
It has been discovered that at a fixed melt index, the higher the melt strength of the overall composition, the lower the neck-in which will be observed. In general it is preferred that the composition will exhibit the following relationship: Melt strength (cN)×Melt Index (g/10 min) will be greater than or equal to 24. Preferably the Melt strength (cN)×Melt Index (g/10 min) is greater than or equal to 26, more preferably greater than 28.
LLDPE in the preferred blends for use in making the compositions of the present invention can be any LLDPE known in the art, and can be varied depending on the physical properties desired, as is generally known in the art.
Another aspect of the present invention is a process for improving extrusion coating performance by using the resins of the present invention.